Cause:
If the shutdown sequence is in progress for UPS's powering the ESXi hosts and then a critical event occurs on the Physical UPS powering the PowerChute agent machine, the local OS shutdown gets initiated too early e.g. before all of the hosts have shut down.

Solution:
Upgrade to PCNS version 4.X

Or
Configure a shutdown command file on the Shutdown Settings page in PowerChute with a duration longer than the vCenter Server VM shutdown duration to allow enough time for the vCenter Server VM host to be shut down properly before the local OS is shut down.